1. Field of the Invention
The present invention relates to a direct current-direct current converter (hereinafter referred to as a "DC-DC converter") that stably outputs predetermined direct current (DC) and DC voltages.
2. Description of the Related Art
FIG. 5 shows an example main circuit configuration of a flyback converter (a DC-DC converter) used as a built-in component for switching power supplies and the like. This flyback converter has a transformer T. At the side of a primary coil N1 of the transformer T. there is formed a primary-side circuit K including a switching device Q formed of a transistor device (in the example converter shown in FIG. 5, a MOS-FET (metal oxide semiconductor-field effect transistor) is used). To this switching device Q there is connected a control circuit 1 for outputting a pulse-control signal, as shown in FIG. 2(c), which controls ON/OFF operations performed by the switching device Q. In the side of a secondary coil N2 of the transformer T, there is provided a rectifying and smoothing circuit 2 including a rectifying diode D and a smoothing capacitor C.
When the level of the pulse-control signal applied by the aforementioned control circuit to a gate (G) of the switching device Q is at a high level (H), the switching device Q is turned ON. In a period when the switching device Q is ON, an input voltage "Vin" allows the primary coil N1 and the switching device Q to be conductive, and energy is charged in the primary coil N1.
Thereafter, when the level of the pulse-control signal goes from the high level (H) to the low level (L), the switching device Q is turned OFF, a primary-side pulse, as shown in FIG. 2(a), is generated at the secondary side, and electrical charge is charged in a parasitic capacitance "Coss" arranged between the drain (D) and the source (S). In a period when the switching device Q is OFF, the energy charged in the primary coil N1 is transferred to a secondary coil N2 and is outputted. The outputted energy is then commutation-smoothed in the rectifying and smoothing circuit 2 and a DC signal is outputted to a load 3.
The aforementioned control circuit 1 has a configuration that performs variable control of an ON period (high-level period) of the pulse-control signal to be applied to the switching device Q in order to stabilize a level of the current or voltage which is outputted to the load 3 from the secondary side at a predetermined level. That is, the control operation of the control circuit 1 allows stabilization of the level of the output current or the output voltage at a predetermined level.
The control circuit 1 can be considered to be formed by use of commonly commercially available PWM control ICs (pulse-width modulation control integrated circuits) or PFM control ICs (pulse-frequency modulation control integrated circuits). However, these control ICs are expensive and not suitable for use in a DC-DC converter in which cost reduction is necessary. For this reason, an RCC (ringing choke converter) type DC-DC converter (a self-excitation type DC-DC converter) is widely used.
However, switching operations in the RCC DC-DC converter are slow; therefore, switching loss occurs when the switching device Q turns ON and OFF. Furthermore, in the RCC DC-DC converter, the switching device Q turns ON from a state when electrical charge is charged in the parasitic capacitance "Coss"; therefore, the electric charge in the parasitic capacitance "Coss" discharges and passes through a section between the drain and the source, causing short-circuit at the switch-ON time.
Such switching loss and short-circuit loss increasingly occurs proportional to the switching frequency. This makes it difficult to achieve size reduction of switching power supplies that must meet increasing switching frequency demands.